High temperature superconducting (HTS) materials such as YBa2Cu3O7-δ (YBCO) are rapidly approaching the manufacturing stage in the form of flexible tapes known as coated conductors. This material will be used in a range of electric power applications such as magnets, motors, generators, and transformers. Although YBCO is capable of supporting very high current densities without resistance, this capability rapidly diminishes in the presence of a magnetic field. As most of the potential applications involve magnetic fields, supercurrent levels would be diminished unless this can be overcome. Ultimately, this phenomenon adversely impacts the cost/performance ratio of HTS products thereby raising the barrier to their commercialization.
Numerous investigations have been undertaken, all seeking to address this problem. Each has involved the same basic approach, i.e., to increase the number of flux-pinning defects within the YBCO superconductor material. Flux pinning refers to the ability of imperfections in the superconductor to inhibit the motion of magnetic flux lines. Such motion dissipates energy and creates resistance to electrical current. This negates the benefit of superconductivity and needs to be overcome. Various features have been shown to enhance the current carrying capacity of HTS materials in a magnetic field, e.g., features such as chemical impurities or interruptions in the crystalline structure of the superconductive material.
The use of surface particles has been used in attempts to enhance defects density in YBCO (see, e.g., Huijbregtse et al., Phys. Rev. B, vol. 62, pp. 1338-1349 (2000)) wherein surface particles were added in a separate deposition step thereby adding to the production cost. Similarly, Crisan et al., App. Phys. Lett., vol. 79, no. 27, pp. 4547-4549 (2001) describe sputtering of metallic nanodots of, e.g., silver, onto strontium titanate substrates prior to deposition of superconducting thin films. Also, Matsumoto et al., Physica C, vol. 412-414, pp. 1267-1271 (2004) describe introduction of nanoscale sized yttrium oxide islands onto strontium titanate substrates prior to deposition of superconducting thin films. In a variant process, Nie et al., Supercond. Sci. Technol., vol. 17, pp. 845-852 (2004) describe high temperature processing of a deposited cerium oxide buffer layer to generate cerium oxide nanodots at the buffer surface prior to deposition of superconducting thin films.
After careful experimentation by the present inventors, it has surprisingly been found that the production of beneficial flux-pinning defects in a YBCO layer can be enhanced by modification of the deposition conditions for selected underlying buffer layers. For example, when employing a strontium titanate (STO) buffer layer between the underlying substrate and the YBCO layer, deposition conditions for the buffer layer exist that result in different surface morphologies for the buffer layer. One example of such deposition conditions for the STO buffer is deposition at temperatures lower than would be optimum for production of the highest self-field critical current density (Jc).
It is an object of the present invention to improve the in-field performance of HTS materials, especially thin film YBCO structures. By “in-field” is meant in the presence of an applied magnetic field, whereas the term “self-field” refers to in the absence of an applied magnetic field.